A sash window disposed within a master frame is well known. The master frame is comprised of a pair of opposed vertical guide rails. The guide rails are adapted to slidably guide a pair of sash windows within the master frame. The guide rails are comprised of dual elongated channels, each channel having an elongated slot opening on the inside of the guide rail which extends the length of the channel.
The prior art, such as Westfall, U.S. Pat. No. 5,353,548, discloses a spring assembly to counterbalance the sash window within the master frame. Generally the spring assembly is a leaf spring which is wound into a spiral coil. The coiled spring is attached to the master frame at its upper end and the lower end of the spring is attached to a pivot brake assembly on the sash window. In this configuration, the distance that the sash window can move within the master frame is limited. The sash window cannot be moved upward beyond the position at which the spring is mounted. Additionally, the spring housing must fit entirely within the channel, thus limiting the width and the strength of the counterbalance spring.